This invention relates to preparation of lower molecular weight alkali soluble polymers by aqueous emulsion polymerization. In particular, it relates to preparation of clear, homogeneous, alkali-soluble acrylic addition copolymers.
It has long been desired to prepare low molecular weight, acid-containing polymers in a low pressure, conventional reactor, which exhibit a uniformity of composition normally associated with polymers prepared in solution. Conventional solution polymerization techniques employ large quantities of expensive solvents, which must be recovered by high-temperature stripping for economic operation, which procedure may create safety problems. In addition, until now, it has proven difficult to prepare a homogeneously distributed low molecular weight copolymer product, utilizing emulsion polymerization techniques of a highly water soluble and water insoluble monomer.
Monoalkenyl aromatic and acrylic addition copolymers having a number average molecular weight (M.sub.n) from about 1000 to 8000 and a weight average molecular weight (M.sub.w) from about 3000 to 20,000 have sometimes been designated "solution polymers". Such acid-containing, alkali soluble copolymers are widely utilized in inks, floor finishes, paper coatings, paints and adhesives. For commercial use, these copolymers are often prepared in concentrates of high solids content. Often they are converted to water soluble "resin cuts" by reaction with a suitable base. To maintain satisfactory low viscosity, clarity and color, such "solution" polymers should exhibit a relatively uniform composition and a low polydispersity index (M.sub.w)/(M.sub.n), usually less than about 3, preferably less than about 2.5.
Heretofore, aqueous emulsion systems were unsuitable for preparation of "solution polymers" of homogeneous composition and relatively narrow molecular weight distribution, which were capable of yielding clear, non-viscous ammoniacal solutions (resin cuts). Highly water soluble monomers, especially acrylic acid monomer, tend to partition strongly into the aqueous phase and tend to form an acrylic homopolymer which is either water soluble or might form either as a shell around the water insoluble monomer/copolymer particles or as an induced coagulum. In addition water insoluble monomers, such as styrene, tend to form homopolymers as the core of the polymer particle, since the water soluble monomers tend to prefer the aqueous phase during initiation of polymerization.
For those reasons and others, emulsion copolymerization of a water insoluble monomer with more than about 10 mole percent of water soluble acrylic monomers have mostly proved unsuccessful. Researchers usually can incorporate from about 2 to 8 mole percent of acrylic monomers into such addition polymers by emulsion polymerization. The results are said to suggest that for acrylic acid monomer, its polymerization sites tend to be on the shell surface or subsurface regions of the latex particle, see Jour. Poly. Sci. Vol. 20, 863-874 (1982); Vestn. Slov. Kem. Drus. 29 /2/ p. 91-102 (1982).
To overcome this problem, an approach was developed which promoted the solubility of acrylic acid monomer in the water insoluble comonomer (styrene) phase to yield a higher acrylic acid content copolymer. This approach utilized concentrated salt solutions to reduce the solubility of acrylic acid in the aqueous phase. This proposed process did not succeed owing to the adverse effects on surface and interfacial tension created by the use of concentrated salt solutions as set forth in U.S. Pat. No. 3,862,924.
Other researchers studying emulsion polymerization of compolymer resins have confirmed that significant polymeric initiation takes place in the aqueous phase or on the latex' surface, rather than in the latex' core site, see Jour. Appl. Poly. Sci., Vol. 23, 893-901 (1979) and Jour. Appl. Poly. Sci., Vol. 20, 2583-2587 (1976). Accordingly, conventional emulsion polymerization techniques simply do not permit successful preparation of clear resin cuts of homogeneous copolymers of acrylic acid and insoluble monomers.
When insufficient acrylic monomer is available in the micelle, then homopolymers of water insoluble monomers can form. Such homopolymers (of polystyrene, for example) reduce the clarity of the resulting polymer product and render it useless for many solution polymer applications. When acrylic acid monomer homopolymerizes on the latex' surface, the resulting polymer product forms more viscous resin cuts, which reduces their utility, significantly. Such polymer products are not simply interpolymers of water soluble and water insoluble monomers, but contain blocks of homopolymers of such monomers, rendering the polymer heterogeneous, rather than homogeneous, in composition.
A successful process for preparing homogeneous copolymers of water soluble and water insoluble monomers has been illustrated in U.S. Pat. No. 4,410,673. In order to incorporate substantial amounts of water soluble monomers, such as acrylic acid, in such polymers it was noted that initiation and polymerization in the aqueous phase was to be avoided to prevent emulsion polymerization and that initiation must occur only in the polymer droplet. For that reason a polar, water insoluble cosolvent, usually in amounts from 10 to 50% based on solids and capable of partitioning the water soluble monomers into a polymer droplet, was employed. After use, the cosolvent was subjected to a complex separation process to recover it for reuse.
It was also disclosed that for best results a precharge of both water insoluble and water soluble monomers should be employed in amounts up to the steady state monomers composition (the free monomers composition). It was further disclosed that the addition rate of the balance of the comonomers charge should be less than the polymerization rate to yield the desired products. The addition rate of the comonomers charge was said to be from 1/2 to 3 hours.
Accordingly, a key feature of that process was said to be the use of significant amounts of a polar cosolvent to partition the water soluble monomer into the water insoluble monomer/copolymer droplet in order to prepare polymers having a high water soluble monomer content. Use of such a cosolvent created the need for equipment and for processing time to separate and recover the cosolvent from the process (for reuse) and the need for possible purification of the copolymer product to remove traces of cosolvent contamination.
It has also been disclosed that for semicontinuous bulk copolymerization of styrene and butyl acrylate at low feed rates the composition of the resulting copolymer is generally a function of the monomer feed composition, see Jour. Appl. Polym. Sci., Vol. 20, 1753-1764 (1976).
It is also generally accepted in the art that when precharging comonomers is deemed desirable that at least the slower-reacting monomer, such as styrene monomer, is always included in the precharge to promote the reaction rate. It has also been reported that where water soluble monomer has been precharged (together with water insoluble monomer) that (i) copolymers of high polydispersity and higher molecular weight tend to form and (ii) homopolymers of water soluble monomer form which result in heterogeneous, viscous, copolymers, see ACS Symposium Series 165, Emulsion Polymers & Emulsion Polymerization, pp. 415-436, Basset and Hamielec, 1981.